Process for the production of ethyl tert.-alkyl ethers

ABSTRACT

The present invention is a cyclic process for the preparation of ethyl tert.-alkyl ethers by the reaction of an alcohol, ethanol, with an iso-olefin such as isobutylene or isoamylene wherein an effluent from the reaction zone is separated in a distillation column to provide an overhead effluent stream and a bottoms effluent stream comprising ethyl tert.-alkyl ether and unreacted ethanol. The unreacted ethanol is recovered in an adsorption zone comprising a selective adsorbent selected from the group consisting of zeolite 13X, sodium zeolite Y, alumina, silicalite and mixtures thereof. The invention is useful in recovering unreacted ethanol from the bottoms effluent stream and returning the unreacted ethanol to the reaction zone. The invention reduces the cost of this separation which is complicated by the formation of an azeotrope between the unreacted alcohol and the ether.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Ser. No.201,590, filed Feb. 25, 1994, U.S. Pat. No. 5,401,887.

FIELD OF THE INVENTION

The present invention relates to a process for the production of ethersby the reaction of an alcohol with an isoalkene. More particularly, itrelates to an improved process for the production of ethyl tert.-butylether (ETBE) by the reaction of ethanol with isoalkene. The inventionspecifically relates to improvements in the recovery and recycle to thereactor of unreacted ethanol without loss of ETBE product.

BACKGROUND OF THE INVENTION

The production of gasoline motor fuel requires consideration of thebalance between the specifications provided by the automobilemanufacturers and the concern for the environment as controlled by thegovernmental regulations on automobile emissions. Renewed environmentalawareness and the desire for cleaner air on the part of the public hasencouraged gasoline producers to develop reformulated grades of gasolineto reduce emissions from automobiles. Government has supported thisreformulation initiative with new regulations which will result in theaddition of oxygenates such as alcohols and ethers to the gasoline poolin an effort to reduce the level of CO and hydrocarbon emissionscompared to emissions from conventional gasoline grades. Thereformulated grades of gasoline, often referred to as oxyfuels, mustmeet all the typical gasoline specifications, and in addition mustcontain a minimum amount of oxygen. In the United States, according tocurrent regulations, this oxyfuel must be sold in those areas of thecountry which do not meet minimum standards for ozone pollution.

Automotive gasoline is usually sold by a grade such as regular, orpremium, according to its octane rating. This octane rating is ameasurable quality and is derived from a laboratory measurement ofoctane number. The octane number is a rating of the performance of asample of the gasoline in a standard test engine. Typically, two typesof octane numbers are used to characterize the octane rating (i.e., aresearch octane (RON) and a motor octane (MON). These are determinedseparately according to well-known laboratory methods and averaged(RON+MON)/2 to provide an octane rating for a particular grade ofgasoline.

Oxygen may be added to gasoline in the form of an oxygenate such as analcohol including methanol, ethanol, or isopropanol and the like, or anether including methyl tert.-butyl ether (MTBE), ethyl tert.-butyl ether(ETBE), tert. amyl-methyl ether (TAME), and the like. Oxygenates areadded to the gasoline pool comprising hydrocarbons in amounts such thatthe octane rating and oxygen content of the blend increases, withoutexceeding vapor pressure limits. Vapor pressure is a physical propertywhich reflects the mount of volatile material in the motor fuel. A highvapor pressure can result in hydrocarbon emissions to the atmosphere.Although alcohols such as methanol and ethanol have favorable octanenumbers when blended with other gasoline components, the alcoholsgenerally have a higher vapor pressure than ethers. Therefore, thegasoline producers have sought to increase the oxygen content of fuelsby incorporating more renewable resource materials such as ethanol intothe gasoline by converting the alcohols into ethers by combining thealcohols with C₄ and C₅ iso-olefins over an acid catalyst.

The production of ethers by the reaction of an iso-olefin and an alcoholis a well-known commercial operation. A number of detailed descriptionsof such processes, particularly as they relate to the production ofmethyl tert.-butyl ether (MTBE) appear in the technical and patentliterature. Exemplary of patent disclosures are U.S. Pat. No. 3,726,942issued Apr. 10, 1973, to K. E. Louder; U.S. Pat. No. 4,219,678 issuedAug. 26, 1980, to I. Obenaus et al; U.S. Pat. No. 4,447,653 and U.S.Pat. No. 4,575,567 issued to B. V. Vora on May 8, 1984, and Mar. 11,1986, respectively; and U.S. Pat. No. 4,876,394 issued to M. M. Nagji etal Oct. 24, 1989. These ethers are useful as high octane blending agentsfor gasoline motor fuels by virtue of their high Research Octane Number(RON) of about 120 and their low volatility.

MTBE has become the most commonly used ether for gasoline octaneimprovement. For example, a typical reformulated gasoline grade wouldrequire about 11 volume % MTBE to provide a gasoline containing about2.0 wt % oxygen before reaching a vapor pressure limit. In a similarmanner, if ETBE were used, the resulting blend with about 2.7 wt %oxygen would accommodate about 17 volume % ETBE at the same vaporpressure limit. ETBE has a higher octane value than MTBE and a blendingvapor pressure of about one-half that of MTBE. In addition, ETBE likeMTBE is miscible in gasoline in all proportions, but ETBE has a lowerwater solubility than MTBE, giving ETBE better fungibility in gasolineblends. ETBE is less likely than MTBE to be lost in pipeline transport.The cost of production is a major factor on the use of MTBE over ETBE.Methanol is typically derived from natural gas, while Ethanol isgenerally produced by fermentation of organic material. Givenappropriate favorable price equalization of ethanol relative tomethanol, the goal of encouraging the use of more regenerable materialin the gasoline pool may be achieved. ETBE is produced by anetherification reaction of ethanol and an iso-olefin, such asisobutylene, wherein ethanol is present in an amount in excess of thatrequired for the reaction. Typically, the reactor effluent isfractionated to produce a light stream comprising unreacted hydrocarbonsand an ETBE product stream. Although some of the excess ethanol will bewithdrawn with the unreacted hydrocarbon stream, at least a portion ofthe ethanol will remain in the ETBE product. The ethanol remaining inthe ETBE product results in a loss of ethanol, and this ethanolsignificantly raises the vapor pressure and lowers the octane rating ofthe ETBE product. European Patent No. 542596 discloses the use of acostly and energy intensive extraction and three-stage fractionationscheme to separate the unconverted ethanol from the ETBE. Methods aresought to perform the separation of the ETBE from ethanol in the ETBEproduct in an efficient and low cost manner, without the loss of anyvaluable gasoline blending components.

SUMMARY OF THE INVENTION

It is the objective of the instant invention to provide a process forseparating ETBE and other ethyl tert.-alkyl ethers from unreactedethanol. The advantage of this process is that it provides asubstantially pure ETBE, or ethyl tert.-alkyl ether product, preferablycontaining less than about 10,000 ppm-wt ethanol, more preferablycontaining less than about 5000 ppm-wt ethanol, and most preferablycontaining less than about 100 ppm-wt ethanol. This purified etherproduct adds flexibility to the production and blending of reformulatedgasolines and eliminates the octane and vapor pressure limitationscaused by presence of azeotropic mixtures of ETBE and ethanol in theETBE product.

In one embodiment, the invention is a cyclic process for preparing ethyltert.-alkyl ethers comprising a series of steps. A reaction mixtureformed by combining a feedstream comprising hydrocarbons having from 4to 5 carbon atoms per molecule and containing isoalkene is combined witha near stoichiometric ratio of ethanol with respect to the isoalkene.The reaction mixture is contacted and reacted in a reaction zone toproduce a reaction product effluent comprising ethyl tert.-alkyl ether,at least 10,000 ppm weight unreacted ethanol, and unreacted C₄ -C₅hydrocarbons. The reaction product effluent from the reaction zone isseparated in a distillation column to provide an overhead effluentstream comprising unreacted ethanol and unreacted C₄ -C₅ hydrocarbons,and a bottoms effluent stream comprising ethyl tert.-alkyl ether andunreacted ethanol. The bottoms effluent stream is passed to anadsorption zone containing a selective adsorbent to adsorb ethanol andan ether product stream comprising substantially pure ethyl tert.-alkylether is recovered. The selective adsorbent is regenerated with aregenerant stream to recover the ethanol to provide a recycle streamcomprising ethanol, and the recycle stream is returned to the reactionzone.

In a further embodiment, the present invention is a cyclic process forpreparing ethyl tert.-alkyl ethers comprising a series of steps. Afeedstream comprising hydrocarbons having from 4 to 5 carbon atoms permolecule and containing isoalkene is combined with a near stoichiometricratio of ethanol with respect to the isoalkene to provide a reactionmixture. The reaction mixture is contacted and reacted in a reactionzone to produce a reaction product effluent comprising ethyl tert.-alkylether at least 10,000 ppm weight unreacted ethanol and unreacted C₄ -C₅hydrocarbons. The reaction product effluent from the reaction zone isseparated in a distillation column to provide an overhead effluentstream comprising unreacted ethanol and unreacted C₄ -C₅ hydrocarbons,and a bottoms effluent stream comprising ethyl tert.-alkyl ether andunreacted ethanol. The bottoms effluent stream is passed to anadsorption zone containing a selective adsorbent to adsorb ethanol andrecover an ether product stream comprising substantially pure ethyltert.-alkyl ether. The overhead effluent stream is passed to aseparation zone to provide an unreacted C₄ -C₅ hydrocarbon streamdepleted in ethanol and an unreacted ethanol stream. At least a portionof the unreacted C₄ -C₅ hydrocarbon stream is passed to the adsorptionzone to regenerate the selective adsorbent and to recover a hydrocarbonstream comprising ethanol. The hydrocarbon stream comprising ethanol isrecycled to the separation zone.

In a still further embodiment, the present invention comprises a cyclicprocess for preparing ethyl tert.-alkyl ethers comprising a series ofsteps. A feedstream consisting essentially of hydrocarbons having from 4to 5 carbon atoms per molecule and containing isoalkene is combined witha near stoichiometric ratio of ethanol with respect to the isoalkene toprovide a reaction mixture. The reaction mixture is contacted andreacted in a reaction zone, preferably in the liquid phase, to produce areaction product effluent comprising ethyl tert.-alkyl ether, at least10,000 ppm weight unreacted ethanol, and unreacted C₄ -C₅ hydrocarbons.The reaction product effluent from the reaction zone is separated in adistillation column. The distillation column contains at least a portionof the reaction zone. The distillation column provides an overheadstream comprising unreacted ethanol and unreacted C₄ -C₅ hydrocarbonsand a bottoms effluent stream comprising ethyl tert.-alkyl ether andunreacted ethanol. The bottoms effluent stream is passed to anadsorption zone containing a selective adsorbent. The selectiveadsorbent is selected from the group consisting of zeolite 13X, sodiumzeolite Y, alumina, silicalite, and mixtures thereof to adsorb ethanol.An ether product stream comprising substantially pure ethyl tert.-alkylether is recovered from the adsorption zone. The overhead effluentstream is passed to a separation zone to provide an unreacted C₄ -C₅hydrocarbon stream depleted in ethanol and an unreacted ethanol stream.At least a portion of the unreacted C₄ -C₅ hydrocarbon stream is passedto the adsorption zone to regenerate the selective adsorbent and torecover a hydrocarbon stream comprising ethanol. The hydrocarbon streamcomprising ethanol is recycled to the separation zone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic process flow design illustrating the process ofthe instant invention employing at least a portion of a water washcolumn raffinate to regenerate the adsorption zone.

FIG. 2 is a schematic process flow diagram illustrating an alternateembodiment of the invention employing a portion of the feedstream toregenerate the adsorption zone.

DETAILED DESCRIPTION OF THE INVENTION

The following description of the present process with respect to theproduction of ETBE is made with reference to the flow diagram of thedrawing. In the interest of simplifying the description of theinvention, the process system in the drawing does not contain theseveral conduits, valves, heat exchangers, and the like which in actualpractice would be provided in accordance with the routine skill in theart to enable the process to be carried out on a continuous basis.

Ethanol in the liquid phase enters the reaction zone through line 10 andenters the reaction zone 12 along with a feedstream comprising C₄ -C₅hydrocarbons, preferably in a liquid stream, comprising isoalkenes suchas isobutylene entering through line 14. Advantageously, all fluidstreams introduced into the system have previously been dried to a watercontent of about 1 ppm-wt to about 10 ppm-wt water at the operatingpressure of the reaction zone. Reaction zone 12 is operated at atemperature which in large measure is dependent upon the particularcatalyst employed, but is generally in the range of about 40° C. to 90°C., and using an internal system pressure sufficient to maintain thereaction mixture in the liquid phase. In the present embodiment, thecatalyst is of the ion-exchange resin type and the temperature of thereactor is about 60° C. The feedstream comprises C₄ -C₅ hydrocarbonsincluding butene-1, cis and trans butene-2, butadiene, isobutane,n-butane, and n-amylenes along with isoalkenes such as isobutylene andisoamylenes. Preferably when the isoalkene is isobutylene, isobutyleneis present in an amount of at least 10 mole-% and preferably the nearstoichiometric molar ratio of ethanol to isobutylene in the reactionzone ranges from about 0.95 to about 1.15. The effluent from the reactorcomprises product ETBE, unreacted ethanol, unreacted C₄ -C₅ hydrocarbonsand diethyl ether in addition to other reaction by-products. Thiseffluent is passed through line 16 to distillation zone 18. In someembodiments, the reaction zone may be made up of a first reaction zoneoutside of the distillation zone and a second reaction zone comprisingat least a portion of the reaction zone contained within thedistillation zone. In addition, the first reaction zone may be furthersubdivided into two or more stages with interstage cooling to removeheat and maintain the reaction in the liquid phase. While in thisillustration the reactor and distillation column are represented as twodifferent zones, relatively recent advancements have made possible thecombination of the function of the reactor and the distillation columninto a single apparatus, examples of which are taught in U.S. Pat. No.5,243,102, which is hereby incorporated by reference. For purposes ofthe present invention, either operational mode is suitably employed. Asa result of the distillation zone, a bottoms effluent stream 20comprising ETBE is recovered from the bottom of distillation column 18.The bottoms effluent preferably contains from about 0.5 to about 7weight per cent of ethanol, and more preferably the bottoms effluentstream contains from about 5000 ppm-wt to about 2 wt % ethanol with theremainder being essentially ETBE, and is passed through line 20 toadsorption zone 22 containing a selective adsorbent which selectivelyadsorbs the ethanol. The selective adsorbent may be any of the commonlyused solid adsorbents such as activated alumina, silicalite, silica gelor zeolitic molecular sieves. It has been found that zeolite X and Ymolecular sieves offer particular advantages in adsorbing ethanol. Moreparticularly, it was found that adsorbents such as zeolite sodiumzeolite X, sodium zeolite Y, alumina, silicalite and mixtures thereofprovide particular advantage for the selective adsorption of ethanol inthe presence of ethyl tert.-alkyl ethers. The ether product, which issubstantially pure ETBE, preferably containing less than 10,000 ppm-wt.ethanol, and more preferably containing less than 5000 ppm-wt. ethanol,and most preferably containing less than about 100 ppm-wt ethanol isremoved from the adsorption zone through line 24. The overhead effluentstream from the distillation column 18 comprises about 0.7 to 1.5 wt %unreacted ethanol, unreacted C₄ -C₅ hydrocarbons, 1 to 100 ppm diethylether as well as trace amounts of other volatile by-products. Thecontent of the ethanol in the overhead effluent is dependent upon theformation of an azeotrope with the unreacted C₄ -C₅ hydrocarbons. Thus,the amount of ethanol removed with the overhead effluent is limited bythe formation of the azeotrope, and the remainder of the excess orunreacted ethanol is withdrawn in the bottoms effluent stream. Thisoverhead effluent stream passes through line 26 to a separation zone, orwater wash column 28, which adsorbs the ethanol. The non-adsorbedhydrocarbons, diethyl ether and other highly volatile impurities passthrough water wash column 28 and, depending upon the intendedutilization of this effluent, are optionally passed through line 30 to asecond adsorbent bed (not shown) containing a selective adsorbent toproduce a relatively pure C₄ -C₅ hydrocarbon stream further depleted inethanol. The particular selective adsorbent involved in the secondadsorbent bed is also not a critical feature. Any of the commonly usedsolid adsorbents such as activated alumina, silica gel or zeoliticmolecular sieves can be employed. It has been found that a sodiumzeolite X is well suited to this application. Of the zeolite adsorbents,particularly zeolite 5A, zeolite 13X and zeolite D are preferred. Morepreferably zeolite 13X offers particular advantages in adsorbing traceamounts of oxygenates. In the present embodiment, on a cyclic basis, aportion of the C₄ -C₅ hydrocarbon stream is passed through line 32 tothe adsorption zone 22 as a regenerant for the adsorbent therein. Thespent regenerant hydrocarbon stream comprising ethanol is returned tothe water wash column 28 via lines 34 and 26. The substantially pureethyl tert.-alkyl ether, or in this case ETBE, is recovered via line 24for use in downstream processing or for blending into reformulatedgasoline. A spent water wash stream is recovered from the water washcolumn in line 38. The spent water wash stream 38 may be passed to awater separation zone (not shown) for recovery of additional amounts ofethanol for recycle to the reactor 12 and for regeneration of the washwater for return to line 36. In an alternative operation, the overheadeffluent stream in line 30 may be passed to a second adsorption zone(not shown) containing an adsorbent selective for the further removal ofoxygenates from the overhead effluent, to provide an overhead effluent,containing unreacted C₄ -C₅ hydrocarbons, with an ethanol content ofless than about 100 ppm-wt ethanol, prior to the use of at least aportion of the overhead effluent stream to regenerate the adsorptionzone 22. The remainder of the overhead effluent stream may be passed toan alkylation zone for the production of alkylate or passed to adehydrogenation zone for the production of additional amounts of isoolefin.

A wide variety of catalyst materials has been found to promote theetherification reaction including ion-exchange resins such asdivinylbenzene cross-linked polystyrene ion exchange resins in which theactive sites are sulfuric acid groups; and inorganic heterogeneouscatalysts such as boric acid, bismuth molybdate, and metal salts ofphosphomolybdic acids wherein the metal is lead, antimony, tin, iron,cerium, nickel, cobalt or thorium. Also boron phosphate, blue tungstenoxide and crystalline aluminosilicates of the zeolitic molecular sievetype have also been proposed as heterogeneous catalysts for the reactionof ethanol and isobutylene.

The reaction conditions are not narrowly critical and depend in largepart upon the particular catalyst composition employed. Thus, both vaporphase and liquid phase processes have been proposed in which reactiontemperatures are from about 50° C. to about 400° C., reaction pressuresvary from about atmospheric to about 1.04 MPa (1500 psig) andstoichiometric molar ratios of ethanol to isoalkene range from 0.2:1 toabout 10:1 and preferably, according to a near stoichiometric molarratio ranging from about 0.95 to about 1.15. Thus, the present processmay employ a near stoichiometric ratio of ethanol with respect to theisoalkene. Both batch type and continuous process schemes may besuitably employed. In the present process the reaction can be carriedout in either the vapor phase or the liquid phase, but the liquid phaseis preferred. For reaction zone portions within distillation zones, thereaction proceeds primarily in the liquid phase. Isobutylene is thepreferred isoalkene, although isoamylene may also be employed.

The selective adsorbent for the process of the present invention will beunderstood by those skilled in the art to be any of the well-knownadsorbents for selectively adsorbing ethanol from a mixture thereof withethyl tert.-alkyl ethers such as ETBE, and the adsorbents can beemployed whether in simple or in compound bed, provided only that theseadsorbents exist and that they be maintained at a capacity for adsorbingessentially all of the ethanol from the distillation column bottoms toproduce ETBE in the desired purity. A number of the typical adsorbentssuch as zeolite 4A and 5A were considered for the instant process, butwere found to adsorb the ethanol too strongly to be regeneratedcompletely. Silica gel was also considered, but silica gel was found notto be selective enough to adsorb much of the ethanol in the presence ofETBE. Silicalite was surprisingly found to provide a good capacity forthe adsorption of ethanol with a relatively sharp mass transfer zone. Amixture of sodium zeolite Y in combination with alumina, similar to theadsorbents described in U.S. Pat. No. 4,725,361 to Fleming for theremoval of trihalomethane from aqueous solutions, also was surprisinglyfound to exhibit high capacities for ethanol with a relatively sharpmass transfer zone and with the ability to be regenerated by anon-reactive gas or liquid. It is believed that when the proportion ofthe sodium zeolite Y in the adsorbent mixture, ranges preferably between10 and 40 wt percent of the mixture, and more preferably when theproportion of sodium zeolite Y ranges between 15-30 wt %, the resultingadsorbent becomes isostructural, thus moderating the strength of theadsorbent by redistributing the number of sodium cations coming incontact with the ethanol. This isostructural form permits the adsorbentmixture to retain a high capacity and selectivity for the ethanol, butlowers the strength of the adsorbent mixture to permit the adsorbentadmixture to be desorbed or regenerated with either a gas phase, or aliquid phase regenerant. Zeolite 13X also was found to have a highinitial capacity for ethanol in the presence of ETBE; however, somedegradation of adsorbent capacity was observed following subsequentregeneration. In commercial service, zeolite 13X should provideperformance within an acceptable range. Thus, silicalite, zeolite 13X,sodium zeolite Y, alumina and mixtures thereof are preferred for use asthe selective adsorbent with the instant invention when configuredeither as separate beds or in compound beds having multiple layers ofadsorbents.

FIG. 2 illustrates an alternate embodiment of the instant invention. Theethanol is passed to the reaction zone 112 in line 110. A feedstreamcomprising C₄ -C₅ hydrocarbons including at least some proportion of anisoalkene such as isobutylene or isoamylene is passed to reaction zone112 via feed header 114. The reaction zone 112 contains a catalyst toproduce a reaction product effluent comprising ethyl tert.-alkyl ethersuch as ethyl tert.-butyl ether, which is withdrawn from the reactionzone 112 and passed in line 116 to a distillation column 118. Thedistillation column 118 separates the reaction product effluent into anoverhead effluent stream 126 comprising unreacted C₄ -C₅ hydrocarbonsand a bottoms effluent stream 120 comprising ethyl tert.-alkyl ether andunreacted ethanol. The bottoms effluent stream 120 is passed to anadsorption zone 122 comprising at least two adsorption beds containingan adsorbent as described hereinabove and selective for the adsorptionof ethanol. A substantially pure ethyl tert.-alkyl ether product iswithdrawn in line 124 for subsequent use in downstream processing orgasoline blending to produce reformulated gasoline. The adsorbent zoneis periodically regenerated on a cyclic basis with at least a portion ofthe feedstream withdrawn from the feed header 114 in line 140 and passedto the adsorption zone 122. A spent regenerant stream 142 comprisingunreacted ethanol is recycled to the reaction zone 112 via line 142which returns the recycle stream to the feed header 114 prior to thereaction zone. The distillation column overhead effluent stream 126 ispassed to a water wash column, or separation zone, 128 wherein theoverhead effluent stream is contacted with a water wash stream 136 toprovide an unreacted C₄ -C₅ hydrocarbon stream 130, depleted in ethanolcomprising less than 500 ppm-wt ethanol and a spent water wash stream138.

The temperature within the adsorption beds of the adsorption zone ispreferably within the range, initially, of about 30° C. to 50° C., i.e.,essentially the same as the temperature of the effluent from thefractionation (distillation) tower. The pressure in the beds ispreferably maintained such as to cause the streams being treated toremain in the liquid phase. The regeneration of the beds is accomplishedin the conventional manner by purging, preferably in a directioncountercurrent to the direction of flow through the beds during theadsorption step therein. The purge stream, preferably in the liquidphase, is advantageously of the same or similar composition as the C₄-C₅ hydrocarbon stream feed to the etherification reactor. Thetemperature of the purge stream is not narrowly critical, but should beat least greater than the temperature of the feedstream being treatedduring the adsorption step, and is preferably at least 30° C. to 150° C.higher.

The following examples are only used to illustrate the present inventionand are not meant to be limiting.

EXAMPLES EXAMPLE I

A stainless steel adsorbent column (approximately 6.4 mm (1/4 inch)×10mm (4 inch) was filled with about 1.5 grams of adsorbent pellets havinga particle size of about 177 to about 250 microns (60-80 mesh) and wasemployed in a series of adsorption and regeneration tests to evaluatethe suitability of a series of adsorbents for removing ethanol from ETBEin a liquid solution thereof and regenerating the adsorbent with aheated inert gas (helium) or hydrocarbon vapor (n-hexane). The liquidsolution of ethanol in ETBE was a commercial sample obtained from anETBE production facility with the azeotropic composition comprisingabout 2.2 wt-% ethanol. During the adsorption step, the ethanol/ETBEsolution was pumped with a low flow rate, positive displacement pump(Waters 510 HPLC) through the adsorbent column and 1 cc samples of theadsorption effluent were collected for 10 seconds in sealed vials at 30second intervals for a period of up to about 5 minutes. During theregeneration step with helium, helium gas was passed through the columnwhile gradually heating the column from about ambient temperature toabout 230° C. over a period of about 10 minutes. The passing of thehelium gas at 230° C. was continued for an additional period of 10minutes. The adsorption column was allowed to cool to ambientconditions. The adsorption column was weighed after the adsorption stepand after the regeneration step to determine the amount of materialadsorbed or desorbed. When hydrocarbon vapor regenerant was employed,the hydrocarbon was vaporized at 230° C. and passed through theadsorbent column. The vapor flow was continued for about 60 minuteswhile effluent was collected at periodic intervals. The adsorptioncolumn was then isolated and cooled to ambient conditions. The sampleswere analyzed by gas chromatography and the analyses were plotted withtime at the mean time during which the sample was taken to determine theethanol breakthrough time and the stoichiometric time. The breakthroughtime is determined when the ethanol concentration in the effluentreached 5% of the feed composition and the stoichiometric time is basedon the time when the effluent is at 50% of the feed concentration. Table1 presents the results of this experimental procedure for freshactivated adsorbent. The following adsorbents were considered:silicalite, zeolite 13X, zeolite 4A, zeolite 5A, silica gel and anadsorbent comprising caustic treated alumina and about 13 wt-% sodium Yzeolite. (Alumina/NaY)

                  TABLE 1                                                         ______________________________________                                        FRESH ADSORBENT CAPACITY                                                               Breakthrough                                                                              Stoichiometric                                                                            Weight of                                    Adsorbent                                                                              Loading, wt-%                                                                             Loading, wt-%                                                                             Unused Bed, %                                ______________________________________                                        Silicalite                                                                             6.0         7.2         17                                           13X      10.5        13.0        19                                           4A       1.0         2.3         57                                           5A       4.4         7.9         44                                           Silica Gel                                                                             2.2         4.8         54                                           Alumina/ 9.1         11.1        18                                           NaY                                                                           ______________________________________                                    

These results show that silicalite, 13X and the Alumina/NaY mixturedemonstrate significant capacity (greater than 6.0%) for adsorbingethanol from mixtures of ETBE and ethanol.

The weight of unused bed, WUB, for the tube of Example 1 is determinedexperimentally from the following equation: ##EQU1## The weight ofunused bed, WUB, is a measure of the sharpness of the mass transferzone. The lower the WUB, the more efficient is the use of adsorbent forthe separation in the experimental column. Surprisingly, silicalite, 13Xand the Alumina/NaY mixture displayed the lowest values of WUB and aretherefore preferred for the adsorption of ethanol from mixtures thereofwith ETBE in the present invention.

EXAMPLES II

The adsorption and regeneration steps of Example I were repeatedfollowing the regeneration of the samples of some adsorbents tested inExample I. The regeneration was carried out with helium, an inert gas,at 230° C. as described in Example I. The results following this firstcycle are shown in Table 2.

                  TABLE 2                                                         ______________________________________                                        ADSORBENT CAPACITY AFTER FIRST CYCLE                                                   Breakthrough                                                                              Stoichiometric                                                                            Weight of                                    Adsorbent                                                                              Loading, wt-%                                                                             Loading, wt-%                                                                             Unused Bed, %                                ______________________________________                                        Silicalite                                                                             7.4         8.4         12                                           13X      8.7         11.2        22                                           4A       0.5         1.5         67                                           5A       1.2         2.8         57                                           Silica Gel                                                                             2.3         5.1         55                                           Alumina/ 8.3         10.4        20                                           NaY                                                                           ______________________________________                                    

The adsorbent capacity for silicalite, 13X and the Alumina/NaY mixtureshowed some reduction in cycled capacity with a consistently sharp masstransfer zone as evidenced by the weight of unused bed remaining belowabout 20%. The 4A and 5A zeolites showed significant capacity reductionsafter one cycle, while the relatively weak and non-selective capacity ofthe silica gel adsorbent remained essentially the same with a longtransfer zone.

EXAMPLE III

After the second or third regeneration cycle with helium according tothe procedure described in Example I, a regeneration step using hexanevapor was employed. Pure n-hexane was employed to simulate the use of ahydrocarbon stream more consistent with industrial practice which mayhave some coadsorption effect. The results of this vapor phasehydrocarbon regeneration are shown in Table 3.

                  TABLE 3                                                         ______________________________________                                        ADSORBENT CAPACITY                                                            AFTER N-HEXANE REGENERATION                                                                      Break-   Stoichio-                                                            through  metric                                                      REGEN    Loading, Loading,                                                                             Weight of                                  Adsorbent Cycle #  wt-%     wt-%   Unused Bed, %                              ______________________________________                                        Silicalite                                                                              4        4.9      6.1    20                                         13X       4        7.5      13.9   46                                         Alumina/NaY                                                                             4        8.2      10.2   20                                         ______________________________________                                    

The silicalite results indicated that the hexane regeneration caused aloss of capacity and a lengthening of the mass transfer zone. Becausehexane is a non-polar material which should be a preferred adsorbate forsilicalites, it appears that the ethanol is unable to easily displacethe residual hexane, resulting in a lower breakthrough loading and alonger mass transfer zone.

The use of hexane with 13X restored the capacity of the 13X to freshcapacity. However, the more difficult displacement of the n-hexane byethanol appears to have resulted in a much longer mass transfer zonethan with the helium regeneration of Example II. The use of n-hexane toregenerate the Alumina/NaY mixture restored the capacity without anychange in the mass transfer zone.

EXAMPLE IV

The procedure of Example I for the regeneration with n-hexane wasmodified by not heating the regenerant and employing the n-hexane as aliquid at room or ambient conditions. This test was performed on theAlumina/NaY sample, following the vapor regeneration of Example III. Theamount of ethanol desorbed from the Alumina/NaY material comprises onethird of the adsorption capacity. Thus, liquid phase regeneration of thealumina/NaY adsorbent surprisingly also can be employed but at a reducedcapacity (1/3 of a heated regeneration).

EXAMPLE V

A series of Cases for the etherification of a hydrocarbon feed and thesubsequent recovery of an ethanol free ETBE product are shown in Table2. These were calculated from engineering design consideration and basedon the performance of the 13X adsorbent in Example I. In all cases theregeneration was accomplished with a liquid regenerant at 110° C.followed by cooling, and the ETBE product from the adsorption zonecontained less than about 100 ppm-wt of ethanol. The flow schemesconsidered employed either a separate conventional etherificationreaction zone and a distillation column, or a reaction withdistillation, RWD, scheme wherein at least a portion of theetherification zone is contained in the distillation column.

                                      TABLE 2                                     __________________________________________________________________________                                             REACTOR EFFLUENT                                 REACTION                                                                              STOICHIO-  FEED %    Wt-% Ethanol                         CASE                                                                              FEED    ZONE    METRIC RATIO                                                                             ISOBUTYLENE                                                                             OVERHEAD BOTTOMS                     __________________________________________________________________________    A   FCC     CONV    1.01       15        1.5      1.65                        B   FCC     RWD     1.10       15        1.5      .66                         C   DEHYDRO RWD     1.03       45        0.7      1.6                         D   DEHYDRO RWD     0.95       45        0.7      .67                         __________________________________________________________________________

Case A, Table 2, represents the processing of about 45M metrictonnes/hour (100,000 lb/hr) of a C₄ hydrocarbon feed from an FCC unit ina conventional etherification unit followed by the separation of thereactor effluent in a distillation column as shown in FIG. 2. Thereactor was operated at an olefin conversion of 97%, and the ratio ofethanol to iso-alkene in the feed to the etherification reactor was 1%over the stoichiometric ratio. A three-bed adsorption system wasemployed for the removal of the 1.65% ethanol from the ETBE product,wherein each bed contained about 1.36 metric tons (3000 lb) of 13Xadsorbent. The regeneration of the ethanol adsorption beds wasaccomplished with about a 40% fraction of the feed; although about halfof the distillation overhead could also be employed as the regenerantfollowing a water wash step as shown in FIG. 1.

Case B of Table 2 represents the processing of the same feed as Case Ain an etherification unit wherein the distillation column contains anetherification reaction zone, operating at 98% olefin conversion with astoichiometric ratio of ethanol to iso-olefin of 1.1. The resultingdistillation column bottoms contained about 0.66 wt % ethanol as sent toa three-bed adsorption unit for the removal of the ethanol from the ETBEproduct. Each of the three adsorption beds contained about 0.9 metrictonnes (2,000 lb) of 13X adsorbent. The regeneration of the adsorbentbeds was carried out with about a 27 per cent of the feed, althoughabout 30 percent of the water washed distillation column overhead,comprising the unreacted C₄ -C₅ hydrocarbons, also could be employed asthe regenerant.

In case C, about 91M metric tonnes/hour (200,000 lb/hr) of a hydrocarbonfeed comprising C₄ hydrocarbon derived from a butane dehydrogenationprocess was passed to an etherification reaction zone and a distillationcolumn wherein at least a portion of the distillation column containedsecond etherification reaction zone at an overall olefin conversion of98% and a stoichiometric ratio of ethanol to iso-olefin of about 1.03.The resulting distillation column bottoms comprised about 0.67 wt-%ethanol. The distillation column bottoms was sent to a three-bedadsorption unit wherein each of the adsorption bed contained about 10.4metric tonnes (23,000 lb) of 13X adsorbent. In this case C, almost allof the available feed was required to regenerate the adsorption beds.The results of this case C suggested that for this application, theetherification reaction zone should be operated at a lowerstoichiometric ratio, even slightly sub-stoichiometric.

Case D represented the same feed and reaction zone configuration of CaseC, operated at a sub-stoichiometric ratio of 0.95 and at a correspondingolefin conversion of about 92%. The adsorption zone is the same as thatof Case C; however, in Case D, the adsorption beds may be regeneratedwith essentially all of the water washed distillation column overhead.

What is claimed is:
 1. A cyclic process for preparing ethyl tert.-alkylethers comprising the steps:a) contacting and reacting in a reactionzone, preferably in the liquid phase, a reaction mixture formed bycombining a feedstream comprising hydrocarbons having from 4 to 5 carbonatoms and containing isoalkene with a near stoichiometric ratio ofethanol with respect to said isoalkene to produce a reaction producteffluent comprising ethyl tert.-alkyl ether, at least 10,000 ppm (wt.)unreacted ethanol and unreacted C₄ -C₅ hydrocarbons; b) separating thereaction product effluent from the reaction zone in a distillationcolumn to provide an overhead effluent stream comprising unreactedethanol and unreacted C₄ -C₅ hydrocarbons and a bottoms effluent streamcomprising ethyl tert.-alkyl ether and unreacted ethanol; c) passing thebottoms effluent stream to an adsorption zone containing a selectiveadsorbent to adsorb ethanol and recovering an ether product streamcomprising substantially pure ethyl tert.-alkyl ether; and d)regenerating the selective adsorbent with a regenerant stream to recoverethanol and to provide a recycle stream comprising ethanol, andreturning said recycle stream to said reaction zone.
 2. The process ofclaim 1 wherein the selective adsorbent is selected from the groupconsisting of zeolite 13X, sodium zeolite Y, alumina, silicalite, andmixtures thereof.
 3. The process of claim 2 wherein the selectiveadsorbent is configured as multiple layers of adsorbent.
 4. The processof claim 1 wherein the regenerant comprises at least a portion of thefeedstream.
 5. The process of claim 1 wherein the regenerant comprisesat least a portion of the unreacted C₄ -C₅ hydrocarbons.
 6. The processof claim 1 wherein the isoalkene is selected from the group consistingof isobutylene, isoamylene and mixtures thereof.
 7. The process of claim1 wherein the near stoichiometric ratio of ethanol with respect to saidisoalkene ranges from about 0.95 to about 1.15.
 8. The process of claim1 wherein said distillation column bottom product comprises from about0.5 wt % unreacted ethanol to about 7 wt %.
 9. The process of claim 1wherein said bottoms effluent stream comprises from about 5000 ppm wt toabout 2 wt % ethanol.
 10. The process of claim 1 further comprisingpassing the overhead effluent stream to a separation zone to recover anunreacted ethanol stream and an unreacted C₄ -C₅ hydrocarbon streamsubstantially free of ethanol, said unreacted ethanol stream beingreturned to said reaction zone.
 11. The process of claim 10 wherein saidunreacted C₄ -C₅ hydrocarbon stream substantially free of ethanolcomprises less than 500 ppm-wt ethanol.
 12. The process of claim 1wherein the ether product comprises less than about 5,000 ppm-wtethanol.
 13. The process of claim 1 wherein the ether product comprisesless than about 100 ppm-wt ethanol.
 14. The process of claim 1 whereinat least a portion of said reaction zone is contained in saiddistillation column.
 15. The process of claim 1 wherein said contactingand reaction takes place in the liquid phase.